PTC thermistor and method for manufacturing the same

ABSTRACT

The present invention aims to provide a PTC thermister which uses a conductive polymer having a positive temperature coefficient and has a high withstand voltage and high reliability and in which no failure in electrical connection occurs in side electrode even when a mechanical stress occurs due to the thermal shock by repeated thermal expansion of the conductive polymer sheet. It also aims to provide a method to manufacture the above PTC thermister. To achieve the above purpose, the PTC thermister of the present invention comprises ( 1 ) a laminated body made by alternately laminating conductive polymer sheets and inner electrodes, ( 2 ) outer electrodes disposed on tops and bottoms of said laminated body and ( 3 ) multi-layered side electrodes disposed at the center of both sides of said laminated body and is electrically coupled with said inner electrodes and said outer electrodes. And, a side of laminated body having an area on which a side electrode layer is formed and areas on which side electrode is not formed. A method for manufacturing a PTC thermistor comprises the steps of ( 1 ) forming a laminated body by sandwiching a conductive polymer sheet with metal foils, and then integrating them by heat pressing, ( 2 ) sandwiching the laminated body and conductive polymer sheets from the top and bottom by metal foils, and integrating them by heat pressing. A multi-layered PTC thermistor is obtained by repeating above processes.

FIELD OF THE INVENTION

The present invention relates to the PTC thermistors in which aconductive polymer material having a positive temperature coefficient(PTC) of resistance is employed, and methods for manufacturing the same.

BACKGROUND OF THE INVENTION

PTC thermistors have been commonly used in self-regulating heaters, andare increasingly employed in electronic devices as components to protectagainst overcurrent. Exposure to overcurrent in an electric circuitcauses the conductive polymer sheet inside a PTC thermistor to heat upand expand. This thermal expansion of the conductive polymer sheetincreases the resistance of the PTC thermistor and thus reduces thecurrent to a safer level. There are increasing demands for PTCthermistors that carry high currents, have low resistance, are compactin size, and yield a low voltage drop.

A conventional PTC thermistor is described below.

One known PTC thermistor is disclosed in the Japanese Laid-open PatentNo. S61-10203. This PTC thermistor is created by laminating a pluralityof alternate layers of conductive polymer sheets and metal foils, withside electrodes on opposing sides.

FIG. 10 is a sectional view of a conventional PTC thermistor. In FIG.10, a conductive polymer sheet 1 is made of a high polymer material,such as cross-linked polyethylene, and dispersed conductive particles,such as carbon black. An inner electrode 2 is made typically of a sheetof metal foil, and is sandwiched between the conductive polymer sheets1. The inner electrode 2 is also disposed on the top and bottom of theconductive polymer sheet 1, while leaving a no electrode area 3 at thestarting end, portions of the middle and finishing ends of theconductive polymer sheet 1 as shown. Alternate layers of the innerelectrode 2 and conductive polymer sheet 1 form a laminated body 4. Aside electrode layer 5 forms a leader section, and is disposed at theside of the laminated body 4 so as to be electrically coupled to one endof the inner electrode 2.

However, the conventional PTC thermistor created by laminating theconductive polymer sheet 1 and inner electrode 2 alternately to createlow resistance undergoes repetitive expansion and shrinkage of theconductive polymer sheet 1 when an overcurrent condition is created andalleviated. This may cause failure in connections to the side electrodedue to cracking generated as a result of stresses generated by theexpansion and contraction of the conductive polymer sheet 1.

The present invention aims to provide a highly reliable PTC thermistorwith good withstand voltage which eliminates failure in a connection toa side electrode by cracks, and its manufacturing method.

SUMMARY OF THE INVENTION

The PTC thermistor of the present invention comprises:

A laminated body made by alternately laminating a conductive polymersheet and an inner electrode;

an outer electrode disposed on a top and a bottom of the laminated body;and

a multi-layered side electrode disposed at the center of a side of thelaminated body, and electrically coupled with the inner electrode andthe outer electrode.

A side of the laminated body has:

i) an area on which the side electrode is disposed and

ii) an area on which the side electrode is not disposed.

In a method for manufacturing the PTC thermistor of the presentinvention, the conductive polymer sheet is sandwiched from the top andthe bottom by metal foils and integrated by heat pressing to form thelaminated body. The laminated body is then sandwiched from the top andthe bottom by other conductive polymer sheets, and the laminated bodyand the conductive polymer sheets are sandwiched from the top and thebottom by the metal foils. They are integrated by heat pressing. Theseprocesses are repeated for lamination.

In the PTC thermistor as configured above, a side electrode comprisesmultiple layers and is disposed at the center of the side of thelaminated body so as to be electrically coupled to the inner electrodesand the outer electrodes. In addition, the side of the laminated bodyhas areas with and without the side electrode. This feature reducesmechanical stress in the side electrode at the boundary of the multiplelayers of the side electrode layer even when mechanical stress due tothermal impact is applied to the side electrode through repetitivethermal expansion of the conductive polymer sheet during operation ofthe PTC thermistor. Mechanical stress in the side electrode may also bereduced by extrusion of an expanded conductive polymer sheet to an areawhere the side electrode is not formed. Thus, generation of cracks byconcentrated mechanical stress is avoided, thereby eliminating failurein an electrical connection by cracks. In a method for manufacturing PTCthermistors of the present invention, a process to integrate thelaminated body, conductive polymer sheet, and metal foil by heatpressing is repeated for lamination. This process allows uniformthickness of the conductive polymer sheet in each layer to be achieved.Accordingly, a highly reliable PTC thermistor with good withstandvoltage is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a PTC thermistor in accordance with afirst exemplary embodiment of the present invention.

FIG. 1B is a magnified sectional view of a PTC thermistor in accordancewith the first exemplary embodiment.

FIG. 2 is a magnified sectional view of a surface of a copper foil usedfor an inner electrode of the PTC thermistor.

FIGS. 3A-H illustrate a method for manufacturing the PTC thermistor inthe first exemplary embodiment of the present invention.

FIG. 4A is a sectional view of an example of a crack generated in theside electrode during a thermal impact test.

FIG. 4B is a magnified sectional view at I of FIG. 4A of a crackgenerated in the side electrode during a thermal impact test.

FIG. 5A is a perspective view along line II—II of FIG. 5A of a PTCthermistor in accordance with a second exemplary embodiment of thepresent invention.

FIG. 5B is a magnified sectional view of a PTC thermistor in accordancewith a second exemplary embodiment of the present invention.

FIG. 6A-D illustrate a method for manufacturing the PTC thermistor inaccordance with a third exemplary embodiment of the present invention.

FIG. 7 is a temperature—resistance graph of conductive polymer sheetswith different thicknesses.

FIG. 8 is a withstand voltage characteristic graph against thickness ofa conductive polymer.

FIG. 9 is a perspective view of a PTC thermistor chip in which aprotective film is provided on its entire top.

FIG. 10 is a sectional view of a conventional laminated PTC thermistor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Exemplary Embodiment

A PTC thermistor in a first exemplary embodiment of the presentinvention is described with reference to drawings.

FIG. 1A is a perspective view of the PTC thermistor in the firstexemplary embodiment of the present invention. FIG. 1B is a magnifiedsectional view taken along Line A—A in FIG. 1A. In FIGS. 1A and 1B,conductive polymer sheets 11 a, 11 b, and 11 c are made of a mixedcompound of high density polyethylene, i.e. a crystaline polymer, andcarbon black, i.e. conductive particles. Inner electrodes 12 a and 12 bare made of copper foil, and have nickel protrusions 22 in the form ofswelling on a short stalk on both surfaces. To show the image of aprotrusion, an enlarged sectional view of one side of the foil is shownin FIG. 2.

A protective nickel coating layer 23 is plated over the nickelprotrusions 22. The inner electrodes 12 a and 12 b are sandwichedbetween the conductive polymer sheets 11 a, 11 b, and 11 c,respectively. Outer electrodes 13 a and 13 b made of a copper foil aredisposed on the outermost layers of a laminated body, and have nickelprotrusions in the form of swelling on a short stalk on the contactingsurface to the conducive polymer sheets 11 a and 11 c. A protectivenickel coating layer 23 is plated over the nickel protrusions. A firstside electrode layer 14 a, second side electrode layer 14 b, and thirdside electrode layer 14 c are disposed at the center of both opposingends of the laminated body fabricated by laminating the conductivepolymer sheets 11 a, 11 b, and 11 c, the inner electrodes 12 a and 12 b,and the outer electrodes 13 a and 13 b. The inner electrodes 12 a and 12b and the outer electrodes 13 a and 13 b are electrically coupledalternately to the opposing side electrodes 14. No side electrode layerareas 15 a and 15 b are parts on which the side electrode layer 14 isnot formed. These are disposed on the ends of the laminated body, onwhich the side electrode 14 is formed, at both sides of the sideelectrode 14. The first side electrode layer 14 a is a first nickelplated layer, the second side electrode layer 14 b is a copper platedlayer, and the third side electrode layer 14 c is a second nickel platedlayer. The side electrode 14 is formed by laminating these plated layersin the above order. A first epoxy insulating coating resin layer 16 aand a second epoxy insulating coating resin layer 16 b are disposed onthe outermost layers of the laminated body.

A method for manufacturing the PTC thermistor in the first exemplaryembodiment of the present invention as configured above is describednext with reference to FIG. 3.

First, 35 μm thick copper foil 31 is plated in a Watts nickel bath at acurrent density about 4 times higher (20 A/dm²) than normal plating soas to plate nickel protrusions having heights of 5-10 μm. Then, anapproximately 1 μm thick nickel coating film is plated at normal currentdensity (about 4 A/dm²). The copper foil 31, after being plated with thenickel protrusions and nickel coating film, is patterned by means of adie press. The pattern may also be made by means of a photolithographyand etching process.

Next, 50 wt. % of high density polyethylene of 70 to 90% crystallinity,50 wt. % of furnace black having average particle diameter of 58 nm andspecific surface area of 38 m²/g , and 1 wt. % of antioxidant are mixedand dispersed for about 20 minutes using two roll mills heated to about150° C. to fabricate conductive polymer sheet 32 of about 0.3 mm thick.

Then, as shown in FIG. 3A, the three conductive polymer sheets 32 andtwo patterned copper foils 31 are stacked alternately so as to ensurethat the opening on the copper foil sheets 31 alternately appear atopposite sides. This stacked body is then sandwiched from the top andbottom by plain copper foil sheets 33 that have nickel protrusions and anickel coating layer for protecting the nickel protrusions only on thecontacting surface to the conductive polymer sheets 32.

As shown in FIG. 3B, after stacking the layers, they are heat pressed atabout 175° C., in a vacuum of about 20 torr, and under the pressure ofabout 50 kg/cm² for about 1 minute using a vacuum heat press to make anintegrated laminated body 34.

As shown in FIG. 3C, a plurality of through holes 35 is formed on thelaminated body 34 using a drilling machine. The through holes 35 mayalso be created using a die press. Then, an about 40 Mrad electron beamis applied to the laminated body by electron beam irradiation equipmentto crosslink the high density polyethylene.

Next, as shown in FIG. 3D, 10-20 μm thick nickel film is plated on theentire laminated body 34 including the through hole 35 by dipping thelaminated body 34 in the Watts nickel bath for about 30 minutes atnormal current density (about 4A/dm²). Then, 5-10 μm thick copper filmis plated in a copper sulfate plating bath for about 10 minutes,completing the multi-layered plated film 36. Adding 0.5 vol. % ofwetting agent to the nickel sulfate solution allows a plated layer to beformed uniformly onto the inner wall of the through hole 35. A film withlittle residual stress, which reaches up to 20,000-30,000 psi withconventional plating solution, is thus achieved.

Next, as shown in FIG. 3E, a copper foil 33 on the outermost layer andthe multi-layered plated film 36 are patterned. The following process isemployed for forming the pattern. A dry film is laminated to bothsurfaces of the laminated body 34. After UV exposure of the etchingpattern and development, the plated film is chemically etched using ironchloride, following which the dry film is peeled off. Instead of a dryfilm, an etching resist may also be formed by screen printing.

Next, as shown in FIG. 3F, epoxy resin paste is screen printed onto bothsurfaces of the laminated body 34 except for around the through hole 35.The epoxy resin paste is then thermally cured at 150° C. for 30 minutesto form a protective coating resin layer 37. This protective coatingresin layer 37 may also be formed by laminating an insulation resistfilm and patterning using the photolithography and etching process.

Then, as shown in FIG. 3G, a 5-10 μm thick nickel film 38 is plated onthe top and bottom of the laminated body 34 on the areas where theprotective coating resin layer 37 has not been formed and on the innerwall of the through hole 35, at a current density of about 4A/dm² for 10minutes.

As shown in FIG. 3H, the laminated body 34 is then divided into piecesby dicing. The die press method is also applicable for dividing thelaminated body 34. The laminated body 34 has no side electrode areas 15a and 15 b on its opposing ends. The side electrode is located at thecenter of the ends, and the no side electrode area 39, comprising the noside electrode areas 15 a and 15 b, are provided on both sides of theside electrode layers on both ends of the laminated body 34. The PTCthermistor of the present invention is now completed.

Since the inner electrodes 12 a and 12 b are formed of copper foil, theends of the copper foil constituting the inner electrodes 12 a and 12 bmay be activated easily by pretreatment such as acid washing to form theside electrode 14. This enables inner electrodes 12 a and 12 b to haveimproved connection with the nickel plated first and third sideelectrode layers 14 a and 14 c. The inner electrodes 12 a and 12 b havenickel protrusions 22 on the contacting surface to the conductivepolymer sheets 11 a, 11 b, and 11 c. A nickel coating layer 23 forprotecting the nickel protrusions 22 is also provided. This structureallows the shape of the nickel protrusions 22 to be maintainedthroughout the heat pressing process. The strong adhesion between theconductive polymer sheets 11 a, 11 b, and 11 c,and the inner electrodes12 a and 12 b, the outer electrodes 13 a and 13 b can be created by ananchor effect due to the nickel protrusions 22.

The reliability of the thickness of the side electrode 14, a key part ofthe PTC thermistor in the first exemplary embodiment of the presentinvention as configured and manufactured above, is described next.

The first exemplary embodiment of the present invention is compared withComparison A and Comparison B . The PTC thermistor in this exemplaryembodiment has a three-layered side electrode 14 which comprises a 15 μmfirst nickel plated layer which constitutes the first side electrodelayer 14 a , a 5 μm copper plated layer which constitutes the secondside electrode layer 14 b, and a 5 μm second nickel plated layer whichconstitutes the third side electrode layer 14 c. The PTC thermistor inComparison A has a side electrode layer, a key part, formed by singleplating of 25 μm thick nickel. The PTC thermistor in Comparison B has aside electrode layer, a key part, formed by single plating layer of 25μm thick copper. For the comparison, 30 pieces of each type of the PTCthermistors were mounted on printed circuit boards before the trip cycletest. In the test, a 25 V DC power was connected in series. Anovercurrent of 100 A was supplied for one minute, and then stopped for 5minutes. After 1,000 cycles, 10,000 cycles, and 30,000 cycles of thetrip cycle test, 10 pieces were sampled from each type, and investigatedby cross-sectional observation for the presence of any cracks 40 in theside electrode layer as shown in FIG. 4B.

No cracks were observed after 1,000 or 10,000 cycles in the PTCthermistor in the exemplary embodiment of the present invention. After30,000 cycles, however, a crack was found in 1 of the 10 pieces. Asshown in FIG. 4, this crack had found in the second side electrode layer14 b of the copper plating, and had propagated to a minor degreelaterally along the second side electrode layer 14 b, but not as far asthe boundary. The crack had not reached to the third side electrodelayer 14 c, which is made of the second nickel plating layer.

In case of the PTC thermistor in Comparison A, a crack was found in 2out of 10 pieces after 1,000 cycles. The cracks had reached to within 5μm of where a connection failure would occur. After 10,000 cycles,cracks had caused connection failure in all 10 pieces.

In the case of the PTC thermistor in Comparison B, cracks were found inall 10 pieces after 1,000 cycles. Moreover, connection failure hadoccurred in 4 pieces. After 10,000 cycles, connection failure hadoccurred in all 10 pieces.

The above comparison results indicate that the PTC thermistor in theexemplary embodiment of the present invention can reduce the innerstress in the side electrode. Even though the multi-layered PTCthermistor has greater volumetric expansion, compared to a single-layerstructure, in proportion to the number of laminated layers when thermalexpansion of the conductive polymer sheets 11 a, 11 b, and 11 c occursas a result of self-heating when an overcurrent condition exists. Withregard to volumetric expansion in the lateral direction of the laminatedbody, the expanded conductive polymer is extruded to a part where noside electrode layer is formed. This enables the reduction of stress onthe side electrode layer.

In addition, with regard to volumetric expansion in the verticaldirection of the laminated body, cracks stopped at the boundary betweenthe first side electrode layer 14 a and second side electrode layer 14b, preventing connection failure in the side electrode layer, even whena stress is concentrated on a corner of the side electrode layer. Thisis because the plated layers of the side electrode layer of the PTCthermistor comprise the first side electrode layer 14 a made ofhigh-tensile strength nickel, and the second side electrode layer 14 bformed of ductile copper.

More specifically, the stress concentrated on the corner of the sideelectrode layer may be reduced at the boundary between the first sideelectrode layer 14 a and second side electrode layer 14 b in themulti-layered side electrode. The third side electrode layer 14 c,formed of the second nickel plated layer prevent soldering leachingduring mounting the PTC thermistor onto a printed circuit board 41 withsolder 42. Accordingly, durable electrical connection of the sideelectrode configured by plating three layers of nickel, copper andnickel is confirmed.

Second Exemplary Embodiment

The configuration of a PTC thermistor in a second exemplary embodimentof the present invention is described with reference to the drawings.FIG. 5A is a perspective view and FIG. 5B is a sectional view of the PTCthermistor. In FIGS. 5A and 5B, a conductive polymer sheet 51 is made ofa mixed compound of high density polyethylene, i.e. a crystalinepolymer, and carbon black, i.e. conductive particles. Inner electrodes52 a and 52 b are made of a copper foil, and are laminated alternatelywith the conductive polymer sheet 51. An outer electrode 53 is made of acopper foil. An opening 54 is a space provided near one side electrode55 to divide the inner layer into the inner electrodes 52 a and 52 b.The side electrode 55 is connected to the inner electrodes 52 a and 52 band the outer electrode 53. The opening 54 is created near one sideelectrode 55, and is provided near the alternate side in each layer.

The second exemplary embodiment of the present invention differs fromthe first exemplary embodiment in that the inner electrode is dividedinto two parts, i.e., the inner electrodes 52 a and 52 b by the opening54 at near one side electrode 55. In other words, the inner electrodecomprises longer inner electrode 52 a toward one side electrode layer 55and shorter inner electrode 52 b toward the other side electrode 55.

The PTC thermistor having the three-layered side electrode ismanufactured using the method described in the first exemplaryembodiment. More specifically, a first side electrode layer 14 a is madeof 15 μm thick first nickel plated layer, a second side electrode layer14 b is made of 5 μm copper plated layer, and a third side electrodelayer 14 c is made of a 5 μm thick second nickel plated layer. Then, 30pieces of this type of PTC thermistor are mounted on printed circuitboards. Mounted PTC thermistors are connected to a 25-V DC power inseries, and the trip cycle test applying 100 A overcurrent (ON for 1minute, and OFF for 5 minutes) was implemented. After 1,000, 10,000, and30,000 cycles, 10 pieces were sampled and investigated bycross-sectional observation for the electrical connections to the sideelectrode. No cracks were observed in the PTC thermistor of the presentinvention after 1,000, 10,000, and 30,000 cycles.

In this exemplary embodiment, the inner electrodes 52 a and 52 b areconnected to both side electrode layers 55 on opposing sides of thelaminated body. In addition, the inner electrodes 52 a and 52 b aredivided into two parts by the opening 54 disposed near one sideelectrode layer 55. Elongation of the conductive polymer sheet in avertical direction of the laminated body due to volumetric expansion ofthe conductive polymer sheet 51 during operation is thus prevented bythe inner electrode 52 b connected to the side electrode 55.Accordingly, the stress on corners due to vertical elongation may bereduced.

The present invention has a configuration that the inner electrodes 52 aand 52 b are connected to the side electrode 55 on both opposing ends ofthe laminated body. And the opening 54 disposed near one side electrodelayer 55 divides the inner electrode 52 into the inner electrodes 52 aand 52 b. This configuration enables the prevention of expansion relatedto increase in the thickness of the conductive polymer sheet 51 near theside electrode layer 55, resulting in reducing mechanical stress onelectrical connection to the side electrode 55. Accordingly, electricalconnection of the inner electrodes 52 a and 52 b with the side electrodelayer 55 may be secured.

Furthermore, in the manufacture of the PTC thermistor, the intervalbetween the anode and cathode in the plating bath is reduced to a halffor plating multi-plated layers as the side electrode layer 55. Byreducing the interval between the two plating electrodes, the platingthickness of the corners of the side electrode 55 increased. Sincemechanical stress is likely to be concentrated on corners where theouter electrode and side electrode layer 55 contact, the strength of theplated film of the side electrode layer 55 can be improved by increasingthe thickness of the side electrode layer particularly at the corners.

Third Exemplary Embodiment

A method for manufacturing a PTC thermistor in a third exemplaryembodiment of the present invention is described with reference tosectional views of the PTC thermistor shown in FIGS. 6A to 6D.

FIGS. 6A to 6D show the manufacturing method up to the laminationprocess of a conductive polymer sheet and metal foil, which is a keypart of the PTC thermistor in the third exemplary embodiment of thepresent invention.

As shown in FIG. 6A, a conductive polymer sheet 61 is made of a mixedcompound of 50 wt. % of high density polyethylene of a 70 to 90%crystallinity and 50 wt. % of carbon black having average particlediameter of about 58 nm and specific surface area of about 38 m²/g. Thisconductive polymer sheet 61 is sandwiched between a pair of metal foils62 made of a copper foil having nickel protrusions on both sides andnickel coating layer for protecting the nickel protrusions.

Next, as shown in FIG. 6B, the conductive polymer sheet 61 and the pairof metal foils 62 stacked in the previous process are heat pressed for 1minute at a heating plate temperature of about 175° C. which is about40° C. higher than the melting point of the polymer, in a vacuum ofabout 20 torr, and under a pressure of about 50 kg/cm², so as to make afirst laminated body 63.

As shown in FIG. 6C, the first laminated body 63 is sandwiched from thetop and bottom by a pair of conductive polymer sheets 61. Then they arefurther sandwiched from the top and bottom by a pair of metal foils 62made of copper foils having nickel protrusions and nickel coating layerfor protecting the nickel protrusions.

As shown in FIG. 6D, the first laminated body 63, a pair of conductivepolymer sheets 61, and a pair of metal foils 62 stacked in the previousprocess are heat pressed for 1 minute at a heating plate temperature ofabout 175° C., in a vacuum of about 20 Torr, and under the pressure ofabout 50 kg/cm², so as to make a second laminated body 64.

To increase the number of laminated layers, the processes shown in FIGS.6C and 6D are simply repeated.

The remaining process for manufacturing the PTC thermistor is a processto form a side electrode layer. This is manufactured according to themethod described in the first and second exemplary embodiments.

In the third exemplary embodiment of the present invention, thelaminated body is fabricated by using a conductive polymer sheet with athickness of 0.27 mm. This enables the PTC thermistor having uniform0.25 mm thick conductive polymer layers.

The thickness of the conductive polymer of the PTC thermistor afterlamination is described as follows based on the reliability testresults.

The laminated body was manufactured according to the manufacturingmethod of the present invention, using a conductive polymer sheet with athickness of 0.27 mm before lamination. The thickness of the conductivepolymer sheet in each layer of the laminated body was uniformly close to0.25 mm in all layers.

As for comparison, a PTC thermistor was manufactured using threeconductive polymer sheets with a thickness of 0.27 mm each beforelamination, and four sheets of metal film. Conductive polymer sheets andmetal foils were alternately stacked, and heat pressed together at thesame temperature, in the same vacuum, and under the same pressingconditions as for the third exemplary embodiment of the presentinvention. The thickness of the conductive polymer sheet in each layerof laminated body made according to the comparison manufacturing methodwas, from the bottom layer, 0.21 mm, 0.27 mm, and 0.20 mm respectively.It was found that the outer layer was thinner than the inner layer.

When a number of conductive polymer sheets and metal foil sheets areintegrated by heat pressing at the same time, the heat travels from theouter conductive polymer sheet contacting the heating plate to the innerconductive polymer sheet. Due to the influence of this heat conduction,the outer polymer sheet becomes thinner compared to the inner conductivepolymer sheet in case of simultaneous heat pressing, because of thelower viscosity of the outer conductive polymer sheet compared to thatof the inner conductive polymer sheet.

Next, a comparison of dielectric breakdown behavior is described.

Two types of PTC thermistors manufactured using different laminationmethods as described above were connected to a 50 V DC power supply inseries and subjected to a trip cycle test involving one minute of 100 Aovercurrent followed by five minutes of cut-off. The PTC thermistormanufactured according to the present invention showed no abnormalityafter 10,000 cycles. The PTC thermistor manufactured according to thecomparison method showed dielectric breakdown after 82 cycles.

Dielectric breakdown occurred in the PTC thermistor manufacturedaccording to the comparison method due to variations in the thickness ofthe conductive polymer sheets. FIG. 7 shows a graph illustrating themeasurements of temperature against resistance for different thicknessesof the conductive polymer of the PTC thermistor made of the samesubstances. FIG. 8 shows measurements of the withstand voltage of thePTC thermistors. It is apparent from the results in FIGS. 7 and 8 thatthe thinner conductive polymer has a smaller degree of resistanceincrease and a lower withstand voltage. The results of theaforementioned trip cycle test indicate that the PTC thermistormanufactured according to the comparison method have caused aconcentration of overcurrent on the thinner conductive polymer portions,resulting in dielectric breakdown.

Here, the manufacturing method of the present invention comprises thesteps of: sandwiching a conductive polymer sheet from the top and thebottom by a pair of metal foils; heat pressing the conductive polymersheet and metal foils for forming an integrated laminated body;sandwiching the laminated body from the top and the bottom by theconductive polymer sheets, and further sandwiching these conductivepolymer sheets from the top and the bottom by metal foils; and then heatpressing the laminated body, conductive polymer sheets, and metal foilsfor integration. By repeating these steps, conductive polymers withuniform thickness in all layers can be obtained, achieving a PTCthermistor with good withstand voltage.

Next, a comparison between PTC thermistors provided with and without anickel coating layer on the nickel protrusions which take the form ofswelling on a short stalk, a key part of the present invention, and areformed on the surface of the metal foils is explained.

The method for treating the metal foil surface in the present inventionis as follows. The copper foil 21 is plated in the Watts nickel bath atfour times more current density (20 A/dm²) compared with normal to platenickel protrusions with a height of between 5 and 10 μm. About a 1 μmthick nickel coating film is formed at normal current density (4 A/dm²).

For comparison, copper foil with nickel protrusions without a protectivefilm was manufactured.

The metal foil with nickel protrusions has an anchoring effect betweenthe conductive polymer sheet and the metal foil. The metal foil of thepresent invention which has nickel plating over the nickel protrusionsin the form of swelling on a short stalk showed no deformation of thenickel protrusions caused by pressure during heat pressing. However, themetal foil of the comparison showed deformation in the nickelprotrusions in the form of swelling on a short stalk due to the pressureapplied to them during heat pressing. The shape of the swelling-on-stalknickel protrusions is formed by abnormal deposition during plating.Therefore, these protrusions are fragile. The provision of nickelcoating film thus prevents deformation of the nickel protrusions causedby polymer pressure.

Furthermore, the PTC thermistor of the present invention may be providedwith a protective film, as shown in FIG. 9, over the entire top bychanging the screen printing pattern of the resin which acts as theprotective layer. If there is no electrode, the live part, on a top 91of the PTC thermistor as shown in FIG. 9, the protective layer has theeffect of preventing short-circuiting even if the shielding plate isimmediately over the PTC thermistor.

INDUSTRIAL APPLICABILITY

As described above, the PTC thermistor of the present inventioncomprises a laminated body made by alternately laminating conductivepolymer sheets and inner electrodes; outer electrodes provided on thetop and the bottoms of the laminated body, and a multi-layered sideelectrode provided at the center of sides of the laminated body in a wayso as to electrically connect with the inner electrodes and the outerelectrodes. The sides of the laminated body feature an area with a sideelectrode and an area without a side electrode. The method ofmanufacturing PTC thermistors of the present invention repeats the stepsof forming the laminated body by sandwiching the top and bottoms ofconductive polymer sheet with the metal foil sheets and integrating themby means of a heat pressing; and providing conductive polymer sheets onthe top and bottoms of the laminated body, sandwiching these conductivepolymer sheets with metal foils, and integrating them by a heat pressingfor lamination. With the above configuration, mechanical stress on theside electrode caused by repetitive thermal impact resulting fromthermal expansion of the conductive polymer sheet during operation ofthe PTC thermistor may be reduced at the boundary of the multi-layeredside electrode. At the same time, expanded conductive polymer sheet isextruded to an area where no side electrode layer is formed, alsoreducing the mechanical stress on the side electrode. This is achievedby configuring the multi-layered side electrode, which is electricallycoupled to the inner electrode and outer electrode at the center of thesides of the laminated body. The sides of the laminated body are thusprovided with an area with and without a side electrode. Accordingly,the occurrence of cracks due to concentration of mechanical stress ispreventable, and thus connection failure due to propagation of cracksmay be eliminated. The method for manufacturing PTC thermistors in thepresent invention builds a series of layers by repeating the process ofintegrating the laminated body, conductive polymer sheets, and metalfoils using a heat press. This enables the thickness of conductivepolymer of sheet in each layer to be made uniform. Accordingly, a PTCthermistor with good withstand voltage is obtained.

What is claimed:
 1. A PTC thermistor comprising: a laminated body madeby alternately laminating a conductive polymer shoot and an innerelectrode; outer electrodes, each disposed on a top and a bottom of saidlaminated body; and a multi-layered side electrode electrically coupledwith said inner electrode and said outer electrodes; wherein said innerelectrodes are divided into two parts having different lengths, a longerinner electrode and a shorter inner electrode being alternatelyconnected to one side of said side electrode to restrict expansion ofsaid laminated body, said shorter inner electrode being not longer thansaid outer electrodes.
 2. The PTC thermistor as defined in claim 1,wherein said side electrode comprises a first nickel side electrodelayer, a copper side electrode layer, and second nickel side electrodelayer.
 3. The PTC thermistor as defined in claim 1, wherein the innerelectrode comprises a copper foil with nickel protrusions in the form ofswelling on a short stalk disposed on a top and a bottom surface of saidcopper foil and the outer electrode comprises a copper foil with saidnickel protrusions disposed on a surface of said copper foil whichcontacts a conductive polymer sheet wherein a nickel coating layer isformed to cover all nickel protrusions.